1. Field of the Invention
The present invention relates to naphthalenetetracarboxylic acid diimide derivatives and an electrophotographic photoconductive material comprising the same. More particularly, it relates to naphthalenetetracarboxylic acid diimide derivatives comprising a specific substituent and an electrophotographic photoconductive material having improved electron transporting ability by using the same as an electron transferring material.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a photoconductive material is electrically charged and exposed to an image forming light source to form an electrostatic latent image. Next, the latent image is developed by a toner by applying a development voltage. The developed toner image is transferred to a recording medium such as paper, followed by fixation of the transferred image. Such electrophotography is widely used in various apparatuses such as digital or analog copying machines, printers, facsimiles, etc.
As a photoconductive material of the electrophotographic image forming apparatus, selenium photoconductive materials, amorphous silicone photoconductive materials, etc., were used in the past. However, organic photoconductive materials are widely used now. The organic photoconductive materials are classified into a multi-layered photoconductive material and single-layer photoconductor material. In the multilayered photoconductive material, a charge generating material (CGM) and a charge transferring material (CTM) are separately distributed into different layers of a layered laminate and perform different functions. In the single-layer photoconductive material, a charge generating material (CGM) and a charge transferring material (CTM) are dispersed in a single layer. The multi-layered photoconductive material is mainly used to produce a (−)type organic photoconductive material. The single-layer photoconductive material is mainly used to produce (+)type organic photoconductive material.
The (+)type single-layer organic photoconductive material has advantages in that it generates less ozone, which is harmful to human bodies, and it can be produced at a relatively low production cost since it has a single layer structure. In the (+)type organic photoconductive material, a photosensitive layer comprises an electron transferring material, a hole transferring material, a binder resin and a charge generating material. Since the electron transferring ability of the common electron transferring materials presently used is 100 times less than the hole transferring ability of the hole transferring material, the performance of the organic photoconductive material is largely affected by the electron transferring ability of the electron transferring material. Therefore, among the components contained in the photosensitive layer of the (+)type organic photoconductive material, the electron transferring material is the most important.
Examples of compounds commonly used as the electron transferring material include dicyanofluorenone, 2-nitro-9-fluorenone, 2,7-dinitro-9-fluorenone, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2-nitrobenzothiopene, 2,4,8-trinitrothioxanthrone, dinitroanthracene, dinitroacridine, dinitroanthraquinone, naphthoquinone and 3,5-dimethyl-3′5′-di-t-butyldiphenoquinone. Since the compounds having the dicyanofluorenone and diphenoquinone structures have a weak electron transferring ability, when an organic photoconductive material is produced using these compounds as an electron transferring material, there are problems such as reduction in the charged potential and increase in the exposure potential upon use of a long period of time.
The electron transferring material having the naphthalenetetracarboxylic acid diimide structure is known to have an electron transferring ability superior to the naphthoquinone structure. The electron transferring ability of the electron transferring material having the naphthalenetetracarboxylic acid diimide structure is largely affected by the solubility of the electron transferring material in an organic solvent and compatibility of the electron transferring material with a polymer binder resin.
U.S. Pat. No. 4,442,193 discloses a photoconductive material composition comprising a photoconductive substance and a 1,4,5,8-naphthalenebisdicarboxylic acid diimide derivative compound. U.S. Pat. No. 5,468,583 discloses a photoconductive material comprising a conductive layer, a charge generating layer and a polymer binder having an electron transferring material dispersed therein. The electron transferring material comprises at least one of cyclic bisdicarboxylic acid imide compounds represented by formula 1:
In formula 1, Q1 and Q2 are independently branched chain alkyl, un-substituted straight chain alkyl, unsubstituted cycloalkyl, alkyl-substituted cycloalkyl, unsubstituted straight-chain unsaturated alkyl, aryl, C2 to C20 alkyl, alkoxy or hydrogen, provided that both Q1 and Q2 are not hydrogen, R1, R2, R3 and R4 are, independently, hydrogen, C2 to C4 alkyl, a C2 to C4 alkoxy or halogen.
All of the electron transferring materials disclosed in the above patent comprise naphthalenetetracarboxylic acid diimide derivatives as an electron transferring material. The naphthalenetetracarboxylic acid diimide derivatives are the most expected compounds as an electron transferring material owing to their electron transferring abilities. However, the naphthalenetetracarboxylic acid diimide derivatives known so far are not sufficiently satisfactory in solubility to an organic solvent and compatibility with a binder resin. Also, since the derivatives are apt to crystallize and form precipitates in a photosensitive layer and an electron transferring layer, electrophotographic properties of the photoconductive material may deteriorate.
Meanwhile, when naphthalenetetracarboxylic acid diimide derivative compounds are used in a concentration which can maintain compatibility with the binder resin, the photoconductivity of the photoconductive material may deteriorate.